If you will indulge me I'll just put my pro-silverlight hat on.
We do a lot of UI rapid prototyping in windows presentation foundation (which silverlight is a subset). Then we do the actual games using scaleform, which is flash implementation for Direct3D, GL, etc.
I can say with absolute conviction that WPF is *much* better api for developing UIs.*
All our internal project tools are WPF. It's just a hell of a lot better.
*yes I know silverlight is a lot more limited right now, but 2.0 will significantly improve things.
Of course just like flash there will be a period where developers go 'oooh shiny!' and use it just for the sake of it. It won't be till the end of the year (or the olympics) till we see sites actually really exploit it.